Prior to the present invention, benzopinacole and other tetraarylethanes having the formulas ##STR1## have been used as high temperature free radical initiators for the polymerization of vinyl monomers. Additional free radical initiators useful for vinyl monomer polymerization are the corresponding trialkylsilylether derivatives of benzopinacole, shown by the formula, ##STR2## disclosed by Rudolph et al, U.S. Pat. Nos. 3,391,355 and Vio, 3,792,126. Oligomeric silyl pinacoles also shown to be useful as free radical vinyl monomer initiators are shown by Wolfers et al, Offenlegungsschrift No. 2,632,294 and Reuter et al, Offenlelegungsschrift No. 3,151,444.
There also is shown in U.S. Pat. No. 4,535,174, Free-Radical Initiators and Method for Making, assigned to the same assignee as the present invention and incorporated herein by reference. These free-radical initiators are in the form of silyl ethers of aryl pinacoles based on the reaction of an aryl ketone with a monohalo- or dihalo-silane having at least one olefinically unsaturated organic radical attached to silicon, in the presence of an active metal reducing agent, such as magnesium. The free-radical initiators of U.S. Pat. No. 4,535,174 have been found to be valuable as intermediates in the production of polydiorganosiloxanes having terminal or innerchain silyl pinacole functional groups.
It has now been discovered that cyclic silyl pinacoles having the formula ##STR3## are a class of highly efficient free-radical initiators with potential applications for the polymerization of vinyl monomers, the cure of unsaturated polyesters and for the cross-linking of polyolefins, where R and R.sup.1 are the same or different C.sub.(6-14) aromatic organic radicals, and when joined together and attached to the same carbon atom are selected from divalent aryl radicals having the formula ##STR4## R.sup.4 and R.sup.5 are selected from divalent (C.sub.6-14) aryl organic radicals, R.sup.2 is a monovalent radical selected from C.sub.(1-8) alkyl, C.sub.(1-8) alkenyl, C.sub.(1-8) haloalkyl, C.sub.(1-8) alkoxy, or C.sub.(6-13) aryl, R.sup.3 is a monovalent radical selected from hydrogen and R.sup.2 radicals, X is selected from O, S, CH.sub.2, and ##STR5## and a is 0 or 1.
Radicals included within R and R.sup.1 or Formula 1 are, for example, phenyl, tolyl, xylyl, naphthyl, chlorophenyl, methoxy, nitrophenyl, etc. Radicals included within R.sup.2 are, for example, methyl, ethyl, propyl, butyl, etc.; chloroethyl, methoxy, ethoxy, vinyl, aryl, such as phenyl, tolyl, xylyl, naphthyl, etc. Radicals included within R.sup.3 are, for example, R.sup.2 radicals and hydrogen. Radicals included within R.sup.4 and R.sup.5 are phenylene, tolylene, xylylene, naphthylene, and substituted derivatives thereof.